This invention relates to an electronic scale and, more particularly, to an electronic postage scale, the scale having an improved automatic "zero adjustment" feature, an automatic weight-tracking feature and an automatic initializing feature. In an embodiment to be described, the electronic postage scale is provided with an electronic micro-processor unit for carrying out various operations, including a parcel weighing operation, a calculating operation for calculating the required postage in accordance with the weighed parcel, a selected destination zone and a selected class of shipment, and a displaying operation wherein pertinent information corresponding to the weight and calculated postage of the parcel are displayed.
The postage required to ship a parcel to a desired destination is dependent upon the weight of the parcel, the particular destination zone and the class of shipment by which that parcel is to be shipped. It has been proposed, such as in the disclosure of the aforementioned patent application, to provide an automatic electronic postage scale which includes a weight-measuring device for determining the weight of the parcel, and a keyboard by which an operator can enter pertinent information relating to the selected class of shipment, the destination zone and various extra charges, such as insurance and the like, associated with the shipment of that parcel. Electronic circuitry, preferably in the form of a micro-processor unit, responds to the measured parcel weight and the data which is introduced by the user in operating the keyboard to calculate the proper postage for that parcel. This postage is displayed and, if desired, a suitable postage meter can be operated to fix the correct postage to the parcel.
In the postage scale disclosed in the aforementioned application postage is determined by multiplying the measured parcel weight by the postage rate per unit of weight which is associated with the selected class of shipment and selected destination zone. Consequently, large electronic storage tables which store the postage rate for each incremental weight of a parcel that is to be shipped via each available class of shipment to each possible destination zone need not be provided. This is a significant improvement over earlier postage scales which incorporated such electronic storage tables, such as the postage scale described in application Ser. No. 652,820, filed Jan. 27, 1976 by Gudea. By obviating such electronic storage tables, the quantity of memory capacity can be reduced or otherwise used for carrying out additional operations. Since memory capacity is a significant factor in the cost of electronic postage scales, such cost can be reduced by obviating the need for the heretofore required electronic storage tables. Of course, there are limits to the reduction of such memory capacity, such as if various postage rates are not uniformly linear. In that event, electronic storage tables are needed for the nonlinear postage rate ranges.
It is known to use a load cell in various measuring devices, including postage scales. The use of a load cell in such an environment is described in aforementioned application Ser. No. 816,724, and in U.S. Pat. No. 4,064,954 to Rock. A conventional load cell is formed of strain guages connected in a circuit for producing a weight-indication signal. This advantageously avoids the necessity of providing a complex mechanical assembly consisting of levers, springs, and the like for producing the weight-indicating signal. Although no moving parts are needed, the output signal from the load cell which generally is an analog signal, is highly dependent upon temperature, age of the load cell, and other factors. This dependency results in an output signal whose level fluctuates even if the weight or load placed upon the load cell is fixed. Such fluctuations in the output signal of the load cell can result in corresponding fluctuations in the weight measurement and, consequently, in the postage which is determined for the weighed parcel.
To overcome this drawback in the use of a load cell, it has been proposed, in this and other Rock patents (such as U.S. Pat. Nos. 3,951,221; 4,036,316; 4,043,412; and 4,063,064) to convert the output signal produced by the load cell to digital signals, and then to average those digital signals over a period of time. Such averaging is intended to reduce the effect of a significant fluctuation in one digital signal. Hence, the averaged signals are used as a representation of the weight of the parcel. Typically, in the postage scale described in these patents, the average weight is derived on a so-called rotating basis wherein a preset number of digital weight signals are averaged, with the latest incoming digital weight signal replacing the oldest digital weight signal. These patents also describe a "zero adjustment" operation wherein an accurate zero weight reference is maintained even in the presence of expected load cell drift. In this operation, in the absence of a load upon the load cell, an averaged no-load weight is subtracted from a reference level to produce a net weight. This net weight is compared to zero, and if the absolute value of the net weight is greater than zero but less than some predetermined amount, the net weight is incremented or decremented by a much smaller amount toward zero by incrementing or decrementing the reference level. This process continues in successive sampling times until the net weight is exactly zero.
In application Ser. No. 816,724, load cell drift is taken into account by averaging digital weight signal samples in order to avoid anticipated changes in the weight signal due to fluctuations in the output from the load cell. Also, a zero adjustment operation is carried out by comparing a digital no-load weight signal sample to an immediately preceding digital no-load weight signal sample. If the difference between these samples is less than a predetermined amount, then the latest no-load sample is used as a zero reference level. This zero reference level subsequently is subtracted from a gross digital weight signal to produce a true net parcel weight.
Other proposals to correct for drift in the output of a load cell, and primarily, no-load or zero level drift, are described in U.S. Pat No. 3,916,173 to Williams, Jr. and also in U.S. Pat. No. 3,986,012, to Loshbough. In the systems described in these patents, an analog offset voltage is applied to an amplifier which amplifies the output signal produced by the load cell. Zero-level drift is compensated by this analog offset voltage.
It is proposed, in accordance with one aspect of the present invention, to provide an improved technique for avoiding deleterious influences upon a weight-measuring operation in a weighing scale due to the inherent fluctuations in a load cell. An improved technique for obtaining a statistical approximation of the actual weight signals is disclosed. Furthermore, an improved automatic zero-adjustment technique, together with a steady-state weight-adjustment technique are provided. In addition, an initializing operation, or "start-zero" operation, is described, wherein an initial zero reference level is determined during a start-up or zero-reset mode.